1. Field of the Invention
The present invention relates to a ferroelectric memory which stores data in an involatile manner by using a ferroelectric capacitor, and more particularly to a series connected TC unit type ferroelectric RAM in which a cell block is constituted by connecting plural units cells each comprised of a ferroelectric capacitor and a cell transistor connected in parallel, in series.
2. Description of the Related Art
The ferroelectric memory has a feature in that it stores data in an involatile manner by using a ferroelectric capacitor. As a cell array of the ferroelectric memory, there have been various types well known and one of them is series-connected TC unit type (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-255483).
A memory cell array of the series connected TC unit ferroelectric RAM has, for example, the structure shown in FIG. 1. The basic operation timing of such a memory cell array at the data read is as shown in FIG. 2.
Problems of the series connected TC unit type ferroelectric RAM will be described below.
FIGS. 3 and 4 show a condition in which read data is “1” in the waveform diagram of FIG. 2, in detail.
FIG. 3 shows a condition in which a word line WL0 in FIG. 1 is selected so that read data “1” is read from a unit cell MO. FIG. 4 shows a condition in which a word line WL7 in FIG. 1 is selected so that read data “1” is read from a unit cell M7.
In FIGS. 3 and 4, the dotted lines expressed with n0, n1 and n7 indicate changes in potential of the nodes n0, n1 and n7 in the ferroelectric memory in FIG. 1.
For data read to the unit cells M0, M7, for example, a plate line PL0 is set to “H” and accompanied by this, the potential of a bit line BL is raised up to a value corresponding to the read data “1”. Thereafter, the potential of the bit line BL and the potential of a reference bit line Ref.BL are compared to each other with a sense amplifier SA.
If the word line WL0 is selected, time t2 to t3 in which the plate line PL0 is changed from “L” to “H” is longer than a case where the word line WL7 is selected.
When the word line WL0 is selected, parasitic capacity and parasitic resistance generated in the nodes n1, n2, . . . n7 within the unit cell are connected to the plate line PL0. Because all these parasitic capacity and parasitic resistance need to be charged to charge the plate line PL0, time t2 to t3 in which the plate line PL0 is turned from “L” to “H” is expanded.
On the other hand, to rewrite “1” data into the unit cells M0, M7, the plate line PL0 is turned to “L” so as to apply a predetermined voltage to the unit cell.
If the word line WL7 is selected, changing of the plate line PL0 from “H” to “L” is terminated in a relatively short time. Time t4 to t5 for charging the node n7 to the potential “H” of the bit line BL, that is, time for rewriting of the “1” data is longer than a case where the word line WL0 is selected.
As for the reason, if the word line WL7 is selected, the parasitic capacity and parasitic resistance generated in the nodes n0, n1, . . . n6 within the unit cell are connected to the node n7 in the unit cell. Because all the parasitic capacities and parasitic resistances need to be charged in order to discharge the plate line PL0 and rewrite the “1” data, the time t4 to t5 necessary for rewriting of the “1” data prolongs.
In this manner, in the series connected TC unit type ferroelectric RAM, time until the plate line is turned to “H” and time necessary for rewriting of the “1” data differ depending on a position of a selected cell unit (word line).
Because according to the conventional data read system, the control timing of the data read is determined presuming the worst case corresponding to such a situation, necessarily the cycle time prolongs, which is a problem to be solved.
For example, if considering the case shown in FIG. 3, when the word line WL0 is selected, the rewriting of the “1” data can be terminated at time t5′. However, a period for rewriting of the “1” data is secured until the time t5 considering the worse case, that is, a case where the word line WL7 is selected.
Further, if considering the case shown in FIG. 4, when the word line WL7 is selected, the plate line can be turned to “H” at time t3′. However, a period for turning the plate line to “H” is secured until time t3 considering a case where the word line WL0 is selected.